Large Deformation Measurement Based On Water Transfer Printing Grating

The water transfer printing grating method is a measurement tool that can effectively analyze large deformation mechanical problems.The method has some advantages,such as strong intuitive results,low cost and strong interference ability.Integrating the basic principles of the moiré method and digital image-processing technology,this paper uses the water transfer printing grating method to carry out two researches:(1)measure the static deformation of three shapes of polyurethane elastic specimens;(2)discuss the dynamic mechanical properties of polyurethane elastic rods.This paper combines the water transfer printing technology with the digital moiré method for the first time,and proposes the water transfer printing grating method.The principle of the water transfer printing grating method is as follows.It applies the digital image gray value subtraction method to obtain the digital moiré fringes,and introduces the phase-shifting algorithm,smoothing filter and phase-unwrapping algorithm into the step of image-processing,which enable data and information of experiment to realize automatic processing.In terms of phase demodulation,this paper stresses least-square iterative algorithm.Through the establishment of computational models and measurement of experiments,the results obtained by least-square iterative algorithm are compared with the results obtained by the four-step phase-shifting algorithm,which demonstrates the practicability of least-square iterative algorithm.Based on this,the plan that measures static deformation is described.The water transfer printing grating method is used to measure the large deformation displacement field of the three-point bending beam,the half disk under compression and the torus under diametrical compression,and the relationship between the density of grating and sensitivity of measurement is discussed.Adjusting the selection of the reference grating,this paper uses the grating of undeformed specimen as the reference grating,employs the computer to move the image of reference grating.The paper gets the full-field displacement information of the half disk,and compares the displacement distribution curves gained by the two methods,which have good consistency.The water transfer printing grating method is applied to measure the deformation of the discontinuous object(torus)in detail,and achieves the three-dimensional displacement distribution map of the torus by the way of digital image-processing technology.In contrast to the calculation results of the finite element model,they show good agreement,which verifies the reliability of the experiment.Targeting at the dynamic problems,this paper makes use of the water transfer printing grating method and the stress wave theory to study the mechanical response of the polyurethane elastic rod under different strain rates.The polyurethane material itself has the super-elastic and viscoelastic characteristics.The material constants such as Young's modulus and Poisson's ratio of the elastic rod subjected to the impact load will have great changes.In this paper,the water transfer printing grating method is applied to measure the stress wave velocity and dynamic elastic modulus of polyurethane elastic rods,which provides an example for this method to solve dynamic problems.